There are a couple of sites out there which give you a good look at the global spam trending. Most of the links in here I found in the following site:
Messagelabs has a weekly overview of virusses, spam and phishing
Commtouch Spam lab has a nice graph with 30 / 100 days or 12 months view
The Distributed Checksum Clearinghouse has MRTG graphs of mail checked and spam ratio over 300 servers with loads of options to search through specific periods
Spamcop has good statistics showing which IP blocks the spam is coming from
Barracuda Central shows you what types of spam / phishing mails are being sent.